Use a hierarchical config format

[randomir]

We could switch from Python's config INI format to YAML (or JSON), enabling easy nesting of options.

For example:

client: qpu
solver:
  num_active_qubits:
    gt: 4000
  topology:
    type: pegasus

This would enable a partial update (PATCH with a diff), e.g:

solver:
  topology:
    type: chimera

Another possible syntax (closer to the current INI format, is TOML):

[defaults]
token = "..."

[qpu-chimera]
solver.topology.type = "chimera"

[dqm-nonbulk.solver]
name__regex = ".*(?<!bulk)$"
supported_problem_types__contains = "dqm"